The present invention relates to an improved hydraulic reaction force apparatus for a power steering system and, more particularly, to a hydraulic reaction force apparatus for producing an appropriate steering force according to vehicle traveling conditions such as a vehicle speed and a steering angle.
In a conventional power steering system for reducing a steering wheel turning force (steering force) in a vehicle, the system must be appropriately controlled according to various vehicle traveling conditions such as a steering force and a steering angle, which are associated with steering operations by a driver, as well as a vehicle speed. In steering during a stationary swing or traveling at a low speed, a large auxiliary steering force is output to allow steering with a small force. However, in traveling at a high speed, generation of a large auxiliary steering force causes the driver to feel anxiety, which is undesirable to optimize driving feelings. In traveling at a high speed, the auxiliary steering force must be small to increase a force required for steering by the driver. The driver then feels steering heavy to a proper extent and thus stability for straight driving is assured. The above steering force control is also required when the steering angle is increased.
In order to satisfy the above needs, a hydraulic reaction force apparatus is known as an apparatus for giving a proper resistance (i.e., a steering reaction force) to the steering wheel by a hydraulic force controlled by various vehicle traveling conditions at high and low speeds. There are proposed various types of hydraulic reaction force apparatuses. A typical example of the hydraulic reaction force apparatus is described in Japanese Patent Laid-Open No. Application 49102092. A reaction force arm extends from a flow path selection rotary control valve on the input shaft side. A pair of reaction force plungers extend/contract on the arm from both sides of the arm on the output shaft side along the rotational direction. A hydraulic reaction force chamber is formed on the side of the outer ends of the plungers. A reaction force hydraulic pressure is properly supplied to the hydraulic reaction force chamber according to a vehicle speed or the like to drive the plungers. A predetermined restriction force acts on the input shaft through the arm, thereby obtaining the proper steering reaction force and hence allowing steering operations corresponding to the traveling conditions.
However, in the conventional structure, the reaction force plungers must be disposed at an output shaft portion spaced apart from the axis. Therefore, the radial size of the reaction force apparatus is inevitably increased. This typically occurs when the pressure-receiving area of the plunger is increased and the hydraulic reaction force capacity is accordingly increased. A power steering apparatus provided with the reaction force apparatus of this type is disposed in a limited space such as a lower portion of the engine room of the vehicle. Therefore, compactness is one of the factors to be overcome.
A conventional compact reaction force apparatus of this type is disclosed in U.S. Pat. No. 4,034,825. Engaging members such as reaction force plungers and balls are supported on the output shaft side and are movable toward the center of the output shaft. The engaging members are fitted in engaging recesses formed in the input shaft, thereby producing a restriction force.
In the conventional reaction force apparatus described above, if the engaging member is a plunger, the distal end of the plunger is brought into sliding contact with the engaging recess. When a hydraulic reaction force is generated, a large sliding resistance is produced between the engaging recess and the distal end of the plunger. Therefore, a frictional force is increased to cause operational instability of the reaction force apparatus. Furthermore, if the pressure-receiving area of the plunger is increased, the radial size of the reaction force apparatus mounting portion is increased to fail to achieve compactness, and the plunger diameter is increased. The length of the guide for guiding the plunger becomes insufficient and a stable operating state cannot be obtained.
If the engaging member comprises a ball, it is difficult to assure oil tightness between the ball and the guide hole. The amount of hydraulic reaction force leakage is increased. Generation of an effective hydraulic reaction force requires a hydraulic source having a high flow rate, thus resulting in an impractical structure. Therefore, a demand has arisen for developing a hydraulic reaction force apparatus which has compactness and operational stability.